Hypochlorite ions in aqueous solution are corrosive to many metals and are highly toxic to aquatic life. Industrial waste streams containing aqueous hypochlorite are produced by many processes such as in the manufacture of chlorine-caustic and dry bleach. Before these waste streams can be released into public waters, they require treatment to remove hypochlorite ions.
Various methods including photochemically-, thermally-, and chemically-induced decompositions have been proposed for removing hypochlorite from dilute aqueous solutions. For large scale industrial application, chemical methods are most commonly used. Chemical methods, which include the use of hydrogen peroxide, sodium hydrosulfide, hydrochloric acid and sulfur dioxide, or example, are all expensive when very large quantities of dilute aqueous hypochlorite are involved. Waste treatment systems which consume large quantities of these chemicals create a substantial economic burden on processes which they support.
There is a need for an ecologically efficient and economically sound method for decomposing large quantities of dilute hypochlorite. One basis for such a system is the decomposition of hypochlorite by heterogeneous fixed-bed catalysts to give chloride ion and oxygen. A number of such catalysts comprising the oxides and hydroxides of iron, copper, magnesium, nickel and cobalt have been described in the literature. Of these catalysts, those prepared from cobalt are the most active.
Because of certain practical drawbacks, fixed-bed catalysts have not seen widespread commercial application for hypochlorite decomposition. For example, the high alkalinity of hypochlorite solution causes the binder support of most tableted and extruded catalysts to disintegrate, reducing the catalyst totally, or in part, to a fine slurry. Because of the problems associated with recovery and recycle of finely divided catalyst particles in aqueous media, this technology has not seen widespread application. Also, when fixed-bed catalysts are exposed to waste solutions containing both calcium ions and hypochlorite, such as waste from dry bleach manufacture, the catalyst rapidly loses activity due to calcium carbonate deposition in the catalyst pores. Reactivation of blinded catalyst is difficult.
Accordingly, it is an object of this invention to provide an efficient and economically sound catalytic method of decomposing hypochlorite contained in aqueous industrial waste streams, including those containing dissolved and suspended calcium salts. A further object of this invention is to provide a catalyst for use in the present invention which is efficient, non-polluting and long-lived.